1. Field of the Invention
The present invention pertains to techniques for making measurements of subsurface conditions in and surrounding a borehole such as a well or the like. More particularly, the present invention relates to method and apparatus for logging a borehole traversing an earth formation to simultaneously determine two or more downhole parameters, such as a transverse dimension of the borehole, the density of the surrounding formation and the density of the borehole fluid, including drilling mud, and provides method and apparatus which can perform this logging generally while drilling the borehole through the formation.
2. Description of the Prior Art
When drilling a borehole in the earth in search of petroleum, it is necessary to obtain as much information as possible regarding the nature and structure of the formations through which the borehole is passing. This information is necessary to enable the drilling operator to determine the progress of the drilling operation and to control its direction so as to intercept the pay zone. In the past, most of the necessary measurements have been made by pre-boring geological surveying techniques and then by wireline logging of the borehole after it has been drilled. This approach has a number of obvious disadvantages including loss of drilling time, the expense and delay involved in tripping the drill string so as to enable the wireline to be lowered into the borehole and both the build up of a substantial mud cake lining the borehole and invasion of the formation by the drilling fluids during the time period between drilling and taking measurements. An improvement over these prior art techniques is the recently developing art of measuring-while-drilling in which many of the characteristics of the formation are determined substantially contemporaneously with the drilling of the borehole. Measuring-while-drilling logging either partly or totally eliminates the necessity of interrupting the drilling operation to remove the drill string from the hole in order to make the necessary measurements by wireline techniques.
In addition to the ability to log the characteristics of the formation through which the drill bit is passing, obtaining this information on a real time basis provides substantial safety advantages for the drilling operation. Change in the density of the drilling fluid or the density of the formation or diameter of the borehole, for instance, would indicate conditions which possibly would require immediate attention of the driller in order to prevent a possible blowout. The decrease in the density of the drilling fluid might indicate influx of gas into the borehole from the surrounding formation. It would therefore be necessary to take prompt corrective action in order to prevent a blowout, for example by changing the density of the drilling fluid. With the previous wireline techniques, tripping of the drill string under these conditions could greatly increase the chances of a blowout occurring.
As mentioned above, one of the more important pieces of information to ascertain downhole is the density of the formation through which the borehole is passing. A known technique incorporates the use of gamma ray density probes which are devices generally incorporating a gamma ray source and at least one gamma ray detector which is shielded from the source and which during operation of the probe counts the photons emanating from the source and interacting with the electrons of the material of the formation primarily by Compton scattering. The percentage of photons emitted from the source which eventually pass to the detector after having undergone Compton scattering through the formation depends upon the density of the formation. The photons reaching the detector are counted by standard associated signal processing and data counting equipment.
One of the major difficulties encountered by the previously known density measuring devices is the requirement that the logging device physically contact the formation at the borehole wall. This requirement was necessitated by the fact that all of the known devices were wireline devices and were employed hours or even days after drilling. During this delay, the drilling fluids can both invade the formation and build up a coating of substantial thickness on the borehole wall, either one of which would directly affect the accuracy of measurements taken. It would not be possible to trip a drill string and lower a wireline device fast enough to avoid invasion and/or mud cake problems. These problems are not as severe in a measuring-while-drilling situation since there most likely is no significant amount of mud cake deposited on the borehole walls in a drilling situation, and measurements would generally be taken within an hour of drilling through a location to be measured. Some of the attempts to compensate for the mud cake problem in wireline measurements have included the use of two different detectors spaced axially along a deployable pad which is pressed into and plows through part of the mud cake. The near detector receiving radiation which is scattered partially from the mud cake provides a mud cake correction to the further spaced detector receiving radiation which has scattered principally from the formation. Such multiple detectors are usually used in combination with a complex collimation scheme to narrowly define the emitted beam and direct it into a specific region of the formation and to receive, at a particular detector, only radiation coming from a particular region of the formation.
An example of a prior art wireline density probe which is claimed to function regardless of the thickness and the chemical composition of the materials that are located between the density probe and the samples is discussed in U.S. Pat. No. 3,846,631. The disclosed technique comprises passing two gamma ray beams from two intermittently operated sources into the formation, receiving the radiation back scattered from each of the two sources by two separate detectors, and building ratios of products of the four separate counting rates in such a manner that the numerical result is an indication of the density of the formation. The two-detector probes must be deployed against the borehole wall, as previously discussed, and the spacing between the detectors is a critical dimension. Any non-uniformity of the formation materials or the borehole wall between the detectors will cause an erroneous result.
There are currently no known patented gamma radiation density detecting devices which operate in a measuring-while-drilling condition.
Prior art calipers are usually mechanical devices which require physical contact with the borehole walls. This requirement is very difficult to meet while drilling without affecting the steering of the bottom hole assembly. Acoustical calipers would be equally difficult to use during drilling due to the noise caused by the drilling itself.
It would be very advantageous to overcome the limitations and inaccuracies of the prior art by having a system including a method and apparatus for measuring the density of formations while drilling a borehole through the formations without the necessity of defining narrow bands of the formation, performing collimation of the radiation, or physically contacting the borehole walls.
The instant invention will obviate the need for additional caliper measuring devices, or assumptions regarding the borehole dimensions, or assumptions regarding the composition of the formation being logged, or information on the formation derived from secondary sources, or assumptions concerning the fluids in the borehole at the time of the measurement.